Cleaning process



April 16, 1963 F. R. SHERLIKER ETAL 3,085,918

CLEANING PROCESS Filed May 16, 1960 INVENTORS fkfi/vc/s 12751427644658 fawn [w 67% [2 1/6 ZZWIQJ MM United States Patent M35918 QLEANENG HZQCESS Francis Raymond Sherliker, Runcorn, and Kenneth Qharles Henry Eilis, Liverpool, England, assignors to imperial Chemical industries Limited, London, England, a corporation of Great Britain Filed May 16, 1966, Ser- No. 29,336 Claims priority, application Great Eritain May 22, 1959 6 Claims. (Cl. l3430) This invention relates to an improved cleaning process adapted for removing from non-absorbent articles such as metal, porcelain, glass or plastics, contaminants consisting mainly of metal or metallic oxides suspended in aqueous, fatty, waxy or soapy media.

There is, of course, a known technique, well established and widely used in the metal working industries, of removing oily and greasy contaminants by treating the metal parts with powerful grease solvents, particularly the relatively non-toxic chlorinated hydrocarbons such as trichloroethylene, perchloroethylene and methyl chloroform (lzlzl-trichloroethane). The procedure is customarily carried out in specially designed plants wherein the metal parts are treated with hot solvent or with its vapour or in some cases first with the one and then with the other. These techniques are remarkably effective in removing from the workpieces oil and grease and oilbound or grease-bound metal swarf formed during the working and shaping operations and other similarly oilbound or grease-bound adventitious contaminants but a prerequisite for success is that the contaminant or at least that part of it which is apt to cause the suspended foreign matter to adhere to the workpiece must be freely and rapidly soluble in the chosen solvent. Unfortunately there are many contaminants which are not so soluble.

Thus, for instance, there are many phases of metal working, for instance, rolling, pressing, spinning and extrusion of metal workpieces, wherein lubricants are used which are not readily removed by the conventional metal degreasing solvents. Some of them, for instance lubricants used in making rolled aluminium strip or extruded aluminium cans, are themselves initially adequately soluble but in use they become contaminated with particles of metal and of metal oxide detached from the metal surface and then become difficult to remove. In many cases the lubricant itself may be adequately removed by simple solvent cleaning but particles of metal dust or swarf and/or metal oxide are left behind and these can only be removed by hand cleaning. Similarly, in the polishing industry many polishing compositions are used which are based on metal oxides, for instance rouges, suspended in fatty, waxy or soapy materials and which are not effectively removed by the metal degreasing solvents. Typical difiicultly removable contaminants are, for instance, those which include metal soaps such as aluminium stearate or zinc naphthenate. Hitherto it has been customary to remove such tenaciously adherent contaminants by treatment with an aqueous alkali, the mechanism of the procedure being, as is believed, that the alkali saponifies at least a portion of the metal soap, whereupon the alkali metal soap so formed exerts its usual and Well-known detergent action to remove and suspend the other constituents of the contaminant. However, as is apparent, such treatments With aqueous alkali must be followed by repeated water rinses to remove residual alkali and saponification products and then by drying treatments; these can be time-consuming and expensive and they can, moreover, themselves produce deleterious effects.

For instance, there are currently produced numerous metal articles which have a bright electroplated or polished surface and the commercial value of such articles is frequently dependent in considerable measure upon the perfection of the surface finish. Examples are, for instance, jewellery, bezels and other parts of watches, cosmetic containers, decorative metal strips for use, for instance, on motor cars, instrument parts and the like. If such articles are cleaned by use of aqueous solutions, rinsed and dried, then unless copious quantities of demineralised water are used for the final rinsing, the result is commercially unacceptable. As the water evaporates during the drying procedure it gathers into droplets wherein the inorganic salts progressively accumulate, so that there eventually appear at the places from which the droplets finally evaporate away spots or stains which can only be removed by the tedious and expensive method of hand wiping and which cannot in some cases be satisfactorily removed even by that means. These spots or stains mar the perfection of a bright metal'finish and can seriously diminish the value of the articles. Alternaitively they can cause serious blistering and other troubles in cases where the prepared surface is then painted, lacquered or varnished and subsequently comes to be stored in humid or hot and humid conditions.

Again there is a growing use in the polishing industry of water-based polishing compositions based largely on metal oxides, for instance cerium and zirconium oxides, suspended in aqueous media. These are employed, for instance, for polishing cutlery and also for optical components such as lenses. These polishing media also are not readily removed by simple treatment with chlori nated hydrocarbons and aqueous washing treatments may be inefficient or otherwise undesirable.

In United Sates Patent No. 3,003,247, issued October 10, '1961, there is described and claimed a method of drying Water-contaminated surfaces of articles of metal or other non-absorbent material which comprises treating the said surfaces with a chlorinated hydrocarbon solvent, for instance, trichloroethylene or perchlorocthylene, containing dissolved therein a small proportion of a cationic surface active agent. By employing this drying process following a conventional cleaning With an aqueous alkaline detergent medium one can efiectively clean and expeditiously dry non-absorbent articles whose surfaces bear the kind of difficultly removable contaminants discussed above. However, such proceeding, though technically satisfactory, is not all that could be desired in that it employs two separate treatments of the articles in two successive baths. Indeed, more than two baths must be employed because there must obviously be an intervening thorough rinsing away of the alkaline detergent solution before the cleaned articles can be passed forward to the stage of solvent drying.

We have now found, surprisingly, that a much improved procedure for removing from the surfaces of nonabsorbent articles tenaciously adherent contaminants which consist mainly of particles of metal or metal oxides suspended in aqueous, fatty, waxy or soapy media and which are not freely and rapidly removed by the conventional rnetal-degreasing solvents is provided by treating the contaminated surfaces preferably at elevated temperature with a two-phase liquid system consisting essentially of a chlorinated hydrocarbon (for instance, trichloroethylene, perchloroethylene, methylene chloride or =lz lzl-trichloroethane) containing dissolved therein 0.00 1% to 1% of its weight of a cationic surface active agent and also containing suspended therein or mixed therewith a proportion of Water not exceeding 5% by weight.

It is not suggested that this procedure will in all cases entirely remove any and every contaminant consisting essentially of metal or metal oxide suspended in an aqueous, fatty, waxy or soapy base but it does represent a considerable advance on the procedures previously available in that in many cases which have hitherto proved intractable, this present process will yield technically acceptable results.

The process is preferably, but not necessarily, carried out at elevated temperature. Some form of agitation is desirable and in most cases this is conveniently provided by the thermal currents set up by the use of a heated, preferably refluxing, solution in which the articles to be cleaned are immersed. Indeed, one very convenient way of carrying out the procedure is to treat the articles with the specified cleaning solution contained in a simple conventional metal degreasing plant which requires only Slight modification in a manner more particularly described hereinafter.

By way of explanation it will be interpolated here that for convenience the two-phase cleaning liquid will be referred to as the cleaning solution, notwithstanding that since it is a two-phase system it is not strictly correct to call it a solution.

The solution to be used in the cleaning bath contains 0.001% to 1% by weight, preferably 0.05% to 0.5% of the surface active agent and a proportion of water up to 5%, preferably 1% to 2%, by weight. No great advantage appears to follow from the use of larger proportions of the cationic surface active agent and indeed in some cases there can be considerable disadvantage as they may lead to the formation of viscous intractable emulsions and this in turn can lead to complications in drying. When the preferred proportions of water and of cationic surface active agent are used, the article can usually be simply removed from the cleaning bath and allowed to dry as it emerges from the vapour zone which is established in the upper part of the plant. The contaminant is left behind in the bath, the amount of dissolved matter carried out in solution in the solvent which adheres to the surface on removal from the bath being for most purposes wholly negligible. Moreover, Where the highest perfection of a bright polished finish is required, a simple dip in a further bath of the pure solvent interposed before the air drying is all that is needed to prevent the formation of even traces of a solid deposit. If unduly high proportions of the cationic surface active agent are used there may be undesirable foaming when the liquor is boiled and if unduly high amounts of both water and cationic surface active agent are used, then undesirably stable and viscous emulsions may be formed. These tend to cling to the workpieces when they are removed from the bath and thus to complicate the rinsing procedure. In the simplest cases adequate rinsing can be provided by simply removing the workpieces from the cleaning bath and holding them for a short time in the zone of heavy vapour that lies above the refluxing liquor; here they act themselves as condensing surfaces and the solvent that condenses thereon drains off and drips back into the bath, so washing the workpieces. In other cases it is desirable to provide a separate rinsing bath of clean solvent. Conveniently this is continuously replenished and purified by feeding it with the solvent that is condensed on the conventional cooling coils located in the upper portion of the treatment vessel. The condensate is first passed to a water separator and the solvent, so separated from the water is returned to the rinsing bath, whence it steadily overflows into the main cleaning bath. If the specified proportions of water and cationic surface active agent are used, a simple brief dip in the rinsing bath, which is sufficiently agitated by being kept boiling, provides an adequate and expeditious final rinsing treatment which will remove any traces of the cationic surface active agent from the cleaned workpieces.

It has been previously proposed to clean and free from fatty or greasy matter articles of rigid material, especially metal, by treating them at raised temperature with an emulsion of water and a volatile fat solvent prepared with an emulsifying agent such as a water-insoluble metal soap, an alkali metal salt of a fatty acid or of a resin acid or of a sulphonated derivative of such an acid or a wetting agent of the type customarily used in the textile industry, and before or after this treatment with the emulsion, treating the articles also with a fat solvent. The emulsifying agents contemplated for use in this process were all surface active agents of an anionic character and the whole tenor of the teaching was that stable aqueous emulsions should be used, it being suggested that various phenols and aliphatic or aromatic amines be added for the specific purpose of stabilising the emulsions. Moreover, the teaching was to use emulsions containing large proportions of or even consisting predominantly of water, a composition specifically illustrated containing 25% to 30% of the volatile fat solvent, 1% to 2% of the emulsifying agent, 0.5% to 1% of a stabiliser and about 70% of water. Such compositions are, of course, oil-in-water emulsions.

By contrast in the process of the present invention we use surface active agents of a radically different character, namely, cationic surface active agents, that is agents whereof the ion possessing the surface active properties is the positive ion or cation, and thereby we secure considerable advantage in that the process is simplified and made more expeditious. Thus, for instance, we are enabled to use smaller proportions of the surface active agent than have been previously proposed; for example, as already indicated, we use 0.001% to 1%, preferably 0.05% to 0.5% of the agent. Secondly, we are able to clean the contaminated articles with a treatment liquor which contains very much less water than has been hitherto proposed. This is a material advantage since in practice the treatment liquor is usually kept at the boil, so that a portion thereof is continuously distilled and a high water content calls for a greater heat input to distil a given volume of liquorour process is therefore more economical of heat. Thirdly, the prior proposals were to work with relatively stable emulsions; we, on the other hand, aim to avoid the formation of anything that could reasonably be described as a stable emulsionso much so that our process cannot properly be referred to as emulsion cleaning. We do have two liquid phases present and during the boiling of the treatment liquor they do become intimately commingled with the water dispersed in the oily phase, but as soon as the heating is cut down so that ebullition ceases the two phases rapidly separate again. In fact, no real emulsion is formed. From this there follows a notable simplification of the rinsing procedure. With the prior proposals, the treated articles, on removal from the cleaning bath, carried with them adherent emulsion which was carried over into the rinsing bath and rapidly contaminated it; this made it necessary to hold the treated articles in the rinsing bath for a relatively prolonged period and also called for frequent filtration or distillation of the emulsion-contaminated rinsing liquor. With our system the carryover from the cleaning bath to the rinsing bath is very small indeed so that there is no need for any filtration or special distillation of the rinsing liquor. The process is indeed conveniently carried out in a conventional two-compartment metal degreasing plant only very slightly modified as will now be described.

Referring to the drawings, the single FIGURE represents somewhat diagrammatically a cross-section of a conventional two-compartment metal dcgreasing plant slightly modified to adapt it for the purposes of the present process. The outer container 1 is divided by a bafile into two separate compartments 2 and 3, each of which is provided with heating means 4 at its base. Compartment 2 is the cleaning compartment which contains the mixture of solvent, cationic surface active agent and water and 3 is the rinsing tank which contains the virtually ptu'e solvent. In the preferred procedure the liquor in each compartment is kept boiling by means of the heater 4. Plates 5 are provided for supporting the work to be treated. The vapours of solvent rising from compartment 3 and of an azeotrope of solvent and water rising from compartment 2 form a vapour zone lying above the liquor levels. These vapours condense on the condensing coils 6 which, lying adjacent to the upper walls of the container, effectively contain the vapour within the system, the normal vapour level being shown by the line '7. Thermostats (not shown) control the operation of the heaters 4 so as to maintain the vapour line 7 in the desired position. The condensate is collected in a trough 8 which runs round the walls below the condensing coils. From the trough 8 the condensate is passed via pipe to a water separator 10 wherein it separates into two layers. The upper layer of water overflows by pipe 11 and is returned at the point 12 to the base of the cleaning compartment 2. The lower layer of solvent returns by the lower pipe 13 to the rinsing compartment 3 whence it overflows by the weir 14 into the cleaning compartment 2. The box-like conduit 15, which may surround two or all four Walls of the plant, is an optional further safety feature. It is maintained under slight vacuum so that in case of any sudden upward surge of vapour arising, for instance, from the introduction of an unusually wet load of work into the plant, any solvent vapour that escapes beyond the condensing coils is drawn off into the ventilating system and so is prevented from contaminating the atmosphere of the factory.

In use, the work to be cleaned is immersed in the cleaning compartment 2 for an appropriate short time (of the order of a few seconds to a minute or so), the thermal agitation of the boiling solvent being adequate to bring it into intimate and effective contact with all parts of even intricately shaped workpieces. The work is then removed from the liquor, optionally held in the vapour zone for a few seconds (where solvent vapour condensing on the work washes it, the liquid falling back into the cleaning compartment) dipped briefly into the rinsing compartment, and then removed slowly through the vapour zone so that it emerges from the plant cleaned, dried and free from water spots or stains.

It will be noted that with the plant arranged in this way the very small amount of cationic surface active agent and of water that may be carried over with the cleaned articles into the rinsing compartment cannot progressively build up to a concentration that might cause difficulty. Any water so carried over will distil out with the solvent in the form of an azeotrope, Will condense with the solvent on the condensing coils, will pass to the water separator and so will be returned to the cleaning bath. Any cationic surface active agent will not, of course, distil out and will initially remain in the rinsing tank. However, since all condensed solvent is returned to this compart ment and overflows continuously into the cleaning compartment, there is a steady flow of clean solvent through the rinsing tank which constantly carries the cationic surface active agent back into the cleaning compartment and prevents any build-up of the agent in the rinsing tank. So far as the rising tank is concerned, the plant is thus self-cleaning. The cleaning compartment does, of course, require cleaning out periodically since any contaminants removed from the work accumulate here. Depending on the nature of the contaminant it may be possible continuously to bleed off a portion of the liquor from this tank, purify it by filtration, if necessary, and distillation, reject the contaminant and return the purified solvent to the system. With simple plants, however, the normal practice is to shut down the plant occasionally, filter and distil the solvent and then start up again.

Some procedures which may be followed in practising the invention are illustrated by the following examples which, however, must not be taken as in any way limiting the invention.

Example 1 The articles to be treated are impact extruded aluminium sans made with the use of a conventional extrusion lubricant based on calcium stearate which (after use) cannot be satisfactorily removed by a thorough treatment with trichloroethylene in a conventional metal degreasing plant; even after such treatment, .the insides of the cans are not fully clean, since on wiping them with tissue paper the paper is visibly soiled.

A number of such cans are immersed for 30 seconds in a boiling bath of trichloroethylene containing 0.25% of cetyl pyridinium bromide and 1% of water contained in the cleaning compartment of a two-compartment plant such as is described above. The cans are then removed from the bath, held momentarily in the vapour zone to allow them to drain, immersed for 30 seconds in boiling clean trichloroethylene in the rinsing compartment, and then removed slowly through the vapour zone so that they emerge from the plant quite dry. They are found to be thoroughly clean and to exhibit a particularly bright lustrous finish. When the insides of the cans are wiped with tissue paper the paper is not soiled.

Example 2 Optical lenses just removed from a polishing head and so contaminated with specks of pitch and coated with an aqueous polishing composition based on cerium and zirconium oxides are immersed for 30 seconds in a refluxing bath of trichloroethylene containing 0.25% of cetyl pyridinium bromide and 1% of water (both percentages being by weight based on the weight of the solvent) contained in the cleaning compartment of a two-compartment plant such as is described above. The lenses are then removed from the bath, held momentarily in the vapour zone to allow them to drain, immersed for 30 seconds in boiling clean trichloroethylene in the rinsing compartment of the plant, again held in the vapour zone to drain and then removed from the plant. They are thus obtained in a much cleaner and brighter condition than following the conventional procedures adopted hitherto, namely hot solvent treatment to remove the pitch followed by an aqueous acid wash to remove the metal oxides.

Example 3 Copper and brass cutlery forms which prior to plating have been polished with a composition containing aluminium oxide suspended in an aqueous medium with the aid of sodium salts of mixed long chain fatty acids and which are encrusted with this material are cleaned in the manner described in Example 1. The articles are thoroughly cleaned and have a particularly bright and lustrous finish; they are suitable for passing forthwith to the plating shop.

Example 4 Complex components fabricated some in steel and some in aluminium alloy and having internally milled and lapped portions, including sections machined at the base of deep blind holes were cleaned in the manner described in Example 1. The contaminant was mainly metal dust and swarf held in the lubricant. After this cleaning the proportion of castings rejected at the inspection was less than 1%, whereas with the ultrasonic cleaning procedure previously employed the reject rate was as high as 10%.

Instead of the cetyl pyridinium bromide used as the cationic surface active agent in the above examples, other members of that class may be used, for instance, other long chain quaternary ammonium salts such as cetyl trimethyl ammonium bromide, long chain dialkyl dimethyl ammonium chlorides or bromides and long chain alkyl dimethyl benzyl-ammonium chlorides, likewise other types of cationic surface active agents, such for instance, as an agent made by condensation of methyl octadecylamine with 4.6 molecular proportions of ethylene oxide. However, cationic surface active agents which have a pronounced emulsifying action should, for obvious reasons, be avoided.

As will be seen from the examples, the time needed for the whole treatment is surprisingly short and may be anything from a matter of seconds up to 1 or 2 minutes it is rarely necessary for it to extend beyond 3 or 4 minutes.

What we claim is:

1. An improved procedure for removing from the surfaces of non-absorbent articles tenaciously adherent contaminants which consist mainly of particles of metal and metal oxides suspended in aqueous, fatty, waxy and soapy media and which are not freely and rapidly removed by the conventional metal-degreasing solvents which comprises treating the contaminated surfaces, with an agitated, intimately commingled, two-phase liquid system consisting essentially of a chlorinated hydrocarbon containing dissolved therein 0.001% to 1% of its weight of a cationic surface active agent and also containing mixed therewith a proportion of water not exceeding 5% by weight of the chlorinated hydrocarbon.

2. Process as claimed in claim 1 wherein the chlorinated hydrocarbon is a member of the group consisting of trichloroethylene, perchloroethylene, methylene chloride and 1 1: l-trichloroethaneL 3. Process as claimed in claim 1 wherein the proportion of cationic surface active agent employed is 0.05% to 0.5% of the weight of the chlorinated hydrocarbon and the proportion of water is 1% to 2% of the weight of the chlorinated hydrocarbon.

4. A process as set forth in claim 1 in which said twophase liquid is maintained at an elevated temperature.

5. A process as set forth in claim 1 including rinsing said articles with a bath of said chlorinated hydrocarbon after immersion in said two-phase liquid system.

6. A process as set forth in claim 1 including exposing said articles to vapor of said chlorinated hydrocarbon after immersion in the two-phase liquid system.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN IMPROVED PROCEDURE FOR REMOVING FROM THE SURFACES OF NON-ABSORBENT ARTICLES TENACIOUSLY ADHERENT CONTAMINANTS WHICH CONSIST MAINILY OF PARTICLES OF METAL AND METAL OXIDES SUSPENDED IN AQUEOUS, FATTY, WAXY AND SOAPY MEDIA AND WHICH ARE NOT FREELY AND RAPIDLY REMOVED BY THE CONVENTIONAL METAL-DEGREASING SOLVENTS, WHICH COMPRISES TREATING THE CONTAMINATED SURFACES, WITH AN AGITATED, INITMATELY COMMINGLED, TWO-PHASE LIQUID SYSTEM CONSISTING ESSENTIALLY OF A CHLORINATED HYDROCARBON CONTAINING DISSOLVED THEREIN 0.001% TO 1% OF ITS WEIGHT OF A CATIONIC SURFACE ACTIVE AGENT AND ALSO CONTAINING MIXED THEREWITH A PROPORTION OF WATER NOT EXCEEDING 5% BY WEIGHT OF THE CHLORINATED HYDROCARBON. 